Polypropylene, having excellent stretching characteristics, insulation property, and voltage resistance, has been widely used for a film of a film capacitor. With the demand for the film capacitor increasing mainly in automobile fields, home electrical appliances fields and the like, in order to meet the need for the size reduction, films used therein have been demanded so as to have further improved breakdown voltage.
As a film for a film capacitor, there have been disclosed, for example, a film comprising a composition mainly composed of a high stereoregularity polypropylene (Patent Document 1), a polymer insulating material formed from a polypropylene having an ash amount of not more than 40 ppm by weight and a chlorine amount of not more than 2 ppm by weight (Patent Document 2), a film comprising a propylene polymer having an aluminum residue amount of less than 25 ppm and a boron residue amount of less than 25 ppm, and a film obtained by adding a polypropylene having a long chain branch as a masterbatch to a high stereoregularity polypropylene and then biaxially stretching the mixture (Patent Document 4).
However, none of the polypropylene with improved stereoregularity, the polypropylene with decreased impurities, and the addition of a specific polypropylene, has been able to provide a film having sufficient breakdown voltage, and thus a capacitor satisfying the market demand has not been provided.
Patent Document 5 discloses a film for a film capacitor which has a thermal shrinkage ratio in the longitudinal direction of the film falling within a specific range, the film being obtained by irradiating the polypropylene film biaxially stretched with an ultraviolet ray or an electron ray, in order to suppress “wrinkle” occurring in a heat treatment carried out during the preparation of the capacitor element and adversely affecting the life of the capacitor, to thereby maintain the performance (breakdown strength) of the capacitor.
However, the polypropylene used in the Example has a stereoregularity of approximately 91%, and thus the method using the polypropylene having a stereoregularity of approximately 91%, even if succeeding in suppressing the “wrinkle”, fails to provide a film capacitor having a sufficient breakdown voltage.